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Here's what you do to achieve the gradients without spraying light coats.
Use the roller to apply your primer coat. Then, use the 4" Sable brush, using the dry brush technique (apply paint to the brush, the wipe 95% of it off, onto a paper towel, the remaining 5% will seem like a "dry" coat of paint. This will allow you to make easy gradients from the lightest gray to the unpainted section in the center.
For painting the darker gradients, mix a few tones of gray, apply the darkest tone first, then the next shade and so on, blending them with a different, absolutely clean brush. You'll love the results.
Use acrylics, they move on the canvas like oil paint, but dry in an hour. So, you'll have time to make corrections.